DISTRIBUTION OF MOLLUSC A. 499 



The different genera of the Mollusca are peculiar to, or most fre- 

 quent in, certain localities, and even species and varieties are known 

 to have their peculiar limits. This fact pervades the entire range of 

 organic beings, from the lowest plants to man. The geographical dis- 

 tribution of The Mollusca is perhaps that best known to science. The 

 labours of Messrs. Louis Agassiz and R. M'Andrew, Dr. J. E. Gray, 

 Professor Edward Forbes, and others, have done much towards 

 giving us a clear idea of their distribution in space. Climate alone is 

 insufficient to account for the distribution of animals : some higher 

 cause rules here. But while we admit this, still we must acknow- 

 ledge that climate exerts considerable influence in modifying species. 



The distribution of the Mollusca may be considered from three 

 points of view. First, as regards space; second, as regards depth; 

 and third, as regards time; the last belongs to geology. 



We shall now survey the principal divisions of the ocean ; the 

 line of demarcation being drawn, not by latitude or longitude, but 

 by genera and species. 



The Mollusca of the arctic seas are well known to show con- 

 siderable analogy with those of the later Tertiary periods of Europe. 

 Hence the great interest connected with their comparison, as it affords 

 provided we are satisfied with this line of argument a proof that 

 an arctic climate formerly existed in temperate regions. It is the 

 northern Drift of which we are speaking. Even when species are 

 found living in Britain identical with those of the arctic regions, still 

 there is often a difference in the form or size of British and arctic 

 specimens; certain species, such as Cyprina Islandica, being com- 

 paratively small in the south of Britain, larger in Shetland, and 

 attaining their greatest size in Iceland. 



The countries included in the arctic molluscan province are 

 Lapland, Iceland, Greenland, the west coast of Davis' Straits, and 

 Behring's Straits. About 200 species are enumerated by the various 

 arctic voyagers, as found in these seas ; of these about one-half are 

 peculiar to them, and the other half are either found living in the 

 temperate regions of Europe, or in their so-called glacial strata. 



The Boreal province includes the North Atlantic, from Nova 

 Scotia to Iceland, and from thence to Faroe, Shetland, and the 

 Norway coast. 



The number of species is very large ; and more than one-half are 

 common both to Scandinavia and the North American coast, while a 

 great number also are found on the British coast. 



The province called Celtic by Professor Edward Forbes embraces 

 the coasts of Britain, Sweden, and Denmark. 



